Interjections are words or phrases used to exclaim or protest or command. They sometimes stand by themselves, but they are often contained within larger structures.
* Wow! I won the lottery!
* Oh, I don't know about that.
* I don't know what the heck you're talking about.
* No, you shouldn't have done that.
Most mild interjections are treated as parenthetical elements and set off from the rest of the sentence with a comma or set of commas. If the interjection is more forceful, however, it is followed with an exclamation mark. Interjections are rarely used in formal or academic writing.